Metal porous bodies having a three-dimensional network structure are widely used in various filters, catalyst supports, electrodes for batteries, and the like. For example, Celmet (registered trademark, manufactured by Sumitomo Electric Industries, Ltd.) composed of nickel is used as an electrode material for batteries, such as nickel metal hydride batteries and nickel cadmium batteries. Celmet is a metal porous body having open pores and is characterized by having a high porosity (90% or more) compared with other porous bodies, such as metal nonwoven fabrics. Celmet is obtained by forming a nickel layer on the surface of the skeleton of a porous resin having open pores, such as polyurethane foam, performing heat treatment to decompose the foamed resin molded body, and further subjecting the nickel to reduction treatment. The nickel layer is formed by applying carbon powder or the like to the surface of the skeleton of the foamed resin molded body to impart electrical conductivity, and then depositing nickel by electroplating.
Aluminum has excellent characteristics in terms of electrical conductivity, corrosion resistance, and lightweightness. Regarding use in batteries, for example, an aluminum foil having a surface coated with an active material, such as lithium cobalt oxide, has been used as a positive electrode of lithium ion batteries. In order to improve the capacity of the positive electrode, it is conceivable to form an aluminum porous body to increase the surface area so that the inside of the aluminum porous body can be filled with the active material. In that case, even if the thickness of the electrode is increased, the active material can be utilized, and the active material utilization ratio per unit area is improved.
Examples of porous aluminum include aluminum nonwoven fabrics in which aluminum fibers are entangled with each other and aluminum foams produced by foaming aluminum. Patent Document 1 discloses a method of producing a foamed metal having many closed cells, the method including adding a foaming agent and a thickening agent to a metal in a molten state, followed by stirring. Patent Document 2 describes a method of producing a metal porous body, in which the production method of Celmet is applied to aluminum, the method including forming a coating film of a metal (copper or the like) capable of forming a eutectic alloy at a temperature not higher than the melting point of aluminum on a skeleton of a foamed resin molded body having a three-dimensional network structure, applying an aluminum paste thereto, and performing heat treatment in a non-oxidizing atmosphere at a temperature of 550° C. to 750° C. to remove the organic component (foamed resin) and to sinter aluminum powder. Furthermore, Patent Document 3 describes a method of producing an aluminum porous body, the method including vapor-depositing aluminum, using an arc ion plating process, on a three-dimensional network-like plastic substrate having an inner communicating space to form a metallic aluminum layer with a thickness of 2 to 20 μm.